1. Field of the Invention
A method consistent with the present invention relates to a method for welding a tape cartridge reel composed of a flange and a flange-integrated reel hub. More specifically, a method consistent with the present invention relates to a method for welding a tape cartridge reel, by which a flange is welded to a flange-integrated reel hub by means of ultrasonic vibration.
2. Description of the Related Art
Magnetic tape cartridges contain a magnetic tape used in such applications as computer recording media and video tapes, and a reel on which a magnetic tape is to be wound is accommodated in them. This reel is composed of a flange and a flange-integrated reel hub. Together they are joined by means of ultrasonic welding.
In recent years, the high information recording density design of magnetic tapes has developed. As a result from this development, magnetic tape drives, which play magnetic tapes, need to control the position of magnetic tapes with high accuracy. To ensure this high level position control, both flanges of the magnetic tape cartridges, which function as guides for winding tapes regularly, are required to have the parallelism of a predetermined level. To maintain this level, a magnetic tape cartridge reel equipped with a restricted rib was conceived (see Japanese Unexamined Patent Application 2000-173229. Herein, this reel is called “tape reel” as appropriate. This restricted rib is aimed at preventing the deflections of both flanges in a welding process.
FIGS. 4A to 4C are cross-sectional views depicting individual processes for welding a conventional tape reel. Note that an upper reel 101 is positioned upwardly in an assembly state, but it is positioned downwardly in this figure.
Referring to FIG. 4A, a tape reel 100 is formed by joining a lower reel 102 and the upper reel 101 together by means of ultrasonic welding.
The upper reel 101 is composed of a reel hub 103 and a flange portion 104. The reel hub 103, on which a magnetic tape is to be wound, is a cylindrical member having a bottom, and the flange portion 104 protrudes outwardly from the upper edge (the lower edge in this figure) of the reel hub 103. The reel hub 103 and the flange portion 104 are formed of an integrally-molded synthetic resin. Near the rim of the reel hub 103, a welding portion 103a that is an annular-shaped groove is formed. In addition, on the rim of the welding portion 103a, a restricted rib 103b on which the lower reel 102 will abut is formed.
The lower reel 102 is a plate shaped member, and has an opening 102a and a welding rib 102b. The opening 102a is formed at the center of lower reel 102. The welding rib 102b has a substantially triangle cross-section, and is formed facing the welding portion 103a of the upper reel 101. Near the rim of the opening 102a, a circular rib 102c that protrudes in the opposite direction of the welding rib 102b is formed. On the circular rib 102c, an ultrasonic horn 105 will abut, and the circular rib 102c thereby receives the ultrasonic vibration from the ultrasonic horn 105. Also, the pressure from the ultrasonic horn 105 is exerted on the welding rib 102b. 
Next, a description will be given below of a method by which the tape reel 100 configured above is assembled. Referring to FIG. 4A again, the lower reel 102 is placed on the reel hub 103. Subsequently, as shown in FIG. 4B, a circular-shaped front surface 105a of the ultrasonic horn 105 vibrates the circular rib 102c of the lower reel 102 by means of an ultrasonic wave, and at the same time, applies pressure to the welding rib 102b. As a result, the welding rib 102b is melted, and is then joined to the welding portion 103a of the reel hub 103. In this welding process, after the welding rib 102b is melted, the lower reel 102 abuts on the restricted rib 103b of the reel hub 103, and is thereby fixed by this restricted rib 103b. 
In the above conventional welding method, there are the following disadvantages.
The pressure of the ultrasonic horn 105, the amplitude of the ultrasonic wave and the welding time are all set beforehand, and the tape reel 100 needs to be welded under these constant conditions. In addition, the ultrasonic horn 105 causes a mechanical delay. Accordingly, it is difficult to control the ultrasonic horn 105 in such a way that it stops at the moment when the lower reel 102 abuts on the restricted rib 103b. 
In the tape reel 100, the circular rib 102c (on which the ultrasonic horn 105 abuts) and the restricted rib 103b (on which the lower reel 102 abuts) are not aligned with each other horizontally.
Therefore, as shown in FIG. 4C, if the ultrasonic horn 105 keeps pressing the circular rib 102c even after the lower reel 102 abuts on the restricted rib 103b of the reel hub 103, then the rib portion of the lower reel 102 may be bent upwardly with respect to the restricted rib 103b as a fulcrum. Consequently, the precision of the parallelism between both flange portions are deteriorated.
The present invention has been conceived, taking the above disadvantages into account. An object of the present invention is to provide a method for welding tape cartridge reels with ensuring a high level of parallelism between both their flange portions.